To a good approximation hadronic interactions partially disintegrating a nucleus preserve the energy-per-nucleon of the primary CR. This feature allows CR interactions with gas to realize the mechanism explored in [1]}, and expanded upon in [2]}, for explaining the observed UHECR spectrum and composition. Fits in Fig. REF  show that both gas- and photon-dominated environments can give an adequate accounting of UHECR data. This figure shows how the quality of fit to UHECR data changes with \(r_\mathrm {esc}\) , controlling the average number of interactions CRs undergo before escape, and \(r_{g\gamma }\) , controlling whether the source environment is gas- (small values) or photon-dominated (large values). Contours in this figure show the regions excluded by CR (white dotted region), EHE neutrino (grey hatched region), and gamma-ray (cyan hatched region) data. Figure REF  shows that while Sibyll2.3c is able to describe the UHECR data even for gas-dominated sources, it prefers photon-dominated sources. EPOS-LHC, though, prefers sources where photon interactions are more comparable to gas interactions, as can be seen in Fig. REF .
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